There has been considerable interest in recent years in developing high energy density batteries with lithium containing anodes. Lithium metal is particularly attractive as the anode of electrochemical cells because of its extremely light weight and high energy density, compared for example to anodes, such as lithium intercalated carbon anodes, where the presence of non-electroactive materials increases weight and volume of the anode, and thereby reduces the energy density of the cells, and to other electrochemical systems with, for example, nickel or cadmium electrodes. Lithium metal anodes, or those comprising mainly lithium metal, provide an opportunity to construct cells which are lighter in weight, and which have a higher energy density than cells such as lithium-ion, nickel metal hydride or nickel-cadmium cells. These features are highly desirable for batteries for portable electronic devices such as cellular phones and laptop computers where a premium is paid for low weight. Unfortunately, the reactivity of lithium and the associated cycle life, dendrite formation, electrolyte compatibility, fabrication and safety problems have hindered the commercialization of lithium cells.
Lithium battery systems generally include a cathode which is electrochemically lithiated during the discharge. In this process, lithium metal is converted to lithium ion and transported through electrolyte to the battery's cathode where it is reduced. In a lithium/sulfur battery, lithium ion forms one of a variety of lithium sulfur compounds, at the cathode. Upon charging, the process is reversed, and lithium metal is plated, from lithium ion in the electrolyte, at the anode. In each discharge cycle, a significant number (e.g., 15-30%) of available Li may be electrochemically dissolved in the electrolyte, and nearly this amount can be re-plated at the anode upon charge. Typically, slightly less lithium is re-plated at the anode at each charge, as compared to the amount removed during each discharge; a small fraction of the metallic Li anode typically is lost to insoluble electrochemically inactive species during each charge-discharge cycle.
This process is stressful to the anode in many ways, and can lead to premature depletion of Li and reduction of the battery cycle life. During this cycling, the Li anode surface can become roughened (which can increase the rate of field-driven corrosion) and Li surface roughening can increase proportionally to the current density. Many of the inactive reaction products associated with overall Li loss from the anode upon cycling can also accumulate on the increasingly roughened Li surface and may interfere with charge transport to the underlying metallic Li anode. In the absence of other degradation processes in other parts of the battery, the per-cycle Li anode loss alone can eventually render the cell inactive. Accordingly, it is desirable to minimize or inhibit Li-loss reactions, minimize the Li surface roughness/corrosion rate, and prevent any inactive corrosion reaction products from interfering with charge transport across the Li anode surface. Especially at higher current density (which is commercially desirable) these processes can lead to quicker cell death.
The separation of a lithium anode from the electrolyte of a rechargeable lithium battery or other electrochemical cell can be desirable for a variety of reasons, including the prevention of dendrite formation during recharging, reaction of lithium with the electrolyte, and cycle life. For example, reaction of a lithium anode with the electrolyte may result in the formation of resistive film barriers on the anode, which can increase the internal resistance of the battery and lower the amount of current capable of being supplied by the battery at the rated voltage. Many different solutions have been proposed for the protection of lithium anodes in such devices, including coating the lithium anode with interfacial or protective layers formed from polymers, ceramics, or glasses, the important characteristic of such interfacial or protective layers being to conduct lithium ions. For example, U.S. Pat. Nos. 5,460,905 and 5,462,566 to Skotheim describe a film of an n-doped conjugated polymer interposed between the alkali metal anode and the electrolyte. U.S. Pat. No. 5,648,187 to Skotheim and U.S. Pat. No. 5,961,672 to Skotheim et al. describe an electrically conducting crosslinked polymer film interposed between the lithium anode and the electrolyte, and methods of making the same, where the crosslinked polymer film is capable of transmitting lithium ions. U.S. Pat. No. 5,314,765 to Bates describes a thin layer of a lithium ion conducting ceramic coating between the anode and the electrolyte. Yet further examples of interfacial films for lithium containing anodes are described, for example, in U.S. Pat. Nos. 5,387,497 and 5,487,959 to Koksbang; U.S. Pat. No. 4,917,975 to De Jonghe et al.; U.S. Pat. No. 5,434,021 to Fauteux et al.; and U.S. Pat. No. 5,824,434 to Kawakami et al.
A single protective layer of an alkali ion conducting glassy or amorphous material for alkali metal anodes, for example, lithium anodes in lithium-sulfur cells, is described in U.S. Pat. No. 6,02,094 to Visco et al. to address the problem of short cycle life.
While a variety of techniques and components for protection of lithium and other alkali metal anodes are known, especially in rechargeable batteries, these protective coatings present particular challenges. Since lithium batteries function by removal and re-plating of lithium from a lithium anode in each charge/discharge cycle, lithium ion must be able to pass through any protective coating. The coating must also be able to withstand morphological changes as material is removed and re-plated at the anode.
Rechargeable (secondary) lithium batteries present a particular challenge in connection with their use with aqueous electrolytes. Water, and hydrogen ions, are particularly reactive with lithium. Such devices, to be successful in achieving long cycle life, will require very good protection of the lithium anode.
Despite the various approaches proposed for forming lithium anodes and forming interfacial and/or protective layers, improvements are needed, especially for lithium anodes designed for use in aqueous and/or air environments.